What would Star Trek V have been like if Leonard Nimoy had played both Spock and his misguided brother Sybok? Answer: Probably still pretty terrible, all other things being equal.

Laurence Luckinbill, who did play Sybok, says that Nimoy was absolutely determined that Spock and Sybok should be twins, and that he should play both roles. Sort of a throwback to Nimoy's role as the bearded Spock in the episode "Mirror, Mirror." And when Nimoy didn't get his way, he sort of took it out on Luckinbill, as the actor tells StarTrek.com:

He did not say one word to me for quite a long time, other than "Hello," because, I found out later, he had really, really pushed hard to have this be a double role, a dual role for him. I don't know if this is absolutely true. That was the scuttlebutt and I got that from very high up in the food chain of information, that Leonard wanted to play Sybok and play Spock. That would have been a tremendous thing, to do that, but since they weren't twins, they cast me. I think that Bill wanted a separate actor, and he was right. We were very different people. The best compliment I got was, in the last scenes, 20 or 25 weeks later, Leonard looked at me and said, "You know, you're terrific in this." I thought that was a great send-off.

The other entertaining part of the interview is where Luckinbill talks about his endless conversations with Shatner over Sybok's motivations and innermost desires — until shooting began and Shatner told Luckinbull to shut up about the motivations now, because it was time to make the damn movie. [StarTrek.com, via Airlock Alpha]